Solvent-Dependent Structural Variation of Zinc(II) Coordination

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Crystal Growth & Design

Solvent dependent structural variation of zinc(II) coordination polymers and their catalytic activity in the Knoevenagel condensation reaction Anirban Karmakar,* Guilherme M. D. M. Rúbio, M. Fátima C. Guedes da Silva,* Susanta Hazra, and Armando J. L. Pombeiro* Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049–001, Lisbon, Portugal. E-mail: [email protected]; [email protected]; [email protected]

Abstract The novel self-assembled zinc(II) coordination polymers [Zn(L)(H2O)2]n (1), [Zn(L)(H2O)2]n·n(formamide) (2), [Zn(L)(H2O)2]n·n(N-methylformamide) (3), [Zn(L)(H2O)(formamide)]n (4) and [Zn3(L)2(formate)2(4,4’bipyridine)3]n·2n(DMF)·2n(H2O) (5) (L = 5-acetamidoisophthalate) have been synthesized and characterized by elemental microanalysis, IR spectroscopy, thermogravimetric analysis and X-ray singlecrystal diffraction. 1, 2 and 3 are 1D coordination polymers that crystallize in monoclinic P21, monoclinic P21/m and triclinic P-1 systems, respectively, and are pseudo-polymorphic supramolecular isomers with 1 having a helical arrangement, and 2 and 3 exhibiting zig-zag type structures containing different guest molecules. Compound 4 crystallizes in the triclinic P-1 space group and is a 1D coordination polymer that exhibits fused 8-membered and 16-membered dimetallic rings. Compound 5 features a two-dimensional network type polymer with trimetallic cores. Compounds 1 – 5 expand to 3D by means of H-bond interactions. These coordination polymers act as effective heterogeneous catalysts, under mild conditions, for the Knoevenagel condensation reaction of different aldehydes with an active methylene compound (malononitrile) and can be recycled without losing activity.

Introduction Coordination polymers represent a class of materials that can be constructed by assembly of metal cations or their clusters as nodes and, usually, anionic organic ligands as linkers.1-11 Their attractive architectures and topological networks account for their wide application in gas storage,12-15 separation,16 catalysis (see below), drug delivery,17,18 embedding of nanoparticles.19 Due to various coordination modes they can be adjusted to satisfy the requirements of particular structural motifs. However, identical chemical components may produce more than one superstructure, known as supramolecular isomers. Supramolecular isomers involve network structures that have identical chemical compositions but differ in their structures.20-23 Pseudo polymorphic supramolecular isomers concern identical frameworks that contain different counterions, solvent or guest molecules.20,22 Similar to polymorphism, this phenomena results when different but energetically similar packing interactions can operate during crystallization; induction by variation of temperature, solvent, conformation of the ligand, etc., has been well established.24-27 A variety of examples of supramolecular isomerism in coordination polymers has been 1 ACS Paragon Plus Environment


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reported28-32 and this study is important not only in producing novel materials with interesting properties but also in developing a fundamental understanding of the factors influencing crystal growth. Many isomeric zinc(II) imidazolates featuring zeolite-like topologies have been formed by using template strategies.21 Two Zn(II) based 2D metal organic frameworks have recently been reported29 which are pseudo-supramolecular isomeric pairs, and two heterometallic coordination polymers32 are good examples of pseudo-polymorphic supramolecular isomers. A number of coordination polymers or metal organic frameworks have been reported as supramolecular isomers, several being based on the coexistence of different guest molecules.24-32 Coordination polymers have recently been applied in heterogeneous catalysis33-38 and as catalysts for several organic reactions,39-44 e.g., alkylation of aromatics,45 carbonyl–ene and Diels–Alder reaction,46 cyanotrimethylsilation,47 isomerization of α-pinene oxide,48 Mukaiyama–aldol reaction.49 Recently, we have reported various coordination polymers which are catalytically active for the oxidation of alkanes, alcohols or olefins.50-61 Acidic and basic polymers can exhibit high catalytic activity in condensation reactions like the Pechmann,62 Knoevenagel,63-76 and Prins reactions.77 Coordination polymers can work as catalysts through two different components, viz. the metal ions, which either provide the coordinatively unsaturated nodes and/or form the active metal sites integrated into the linker ligand, and the ligands. Depending on the nature of the ligands and binding metal ions, coordination polymers can act as Lewis bases or acids in the catalytic medium. Recently, we have been focusing on the rational designing of various types of coordination polymers and discrete complexes using amido carboxylate ligands and different transition metal ions and their catalytic activity in various organic transformations, namely the Henry reaction.78,79 The amido carboxylate ligands are known to form interesting polymeric frameworks with metal ions.78,79 Besides, the presence of carboxylate groups allows slight conformational changes for the potential assembly of different network architectures. Thus, the two main objectives of the current work are as follows: (i) to synthesize Zn(II)-coordination polymers by using 5-acetamidoisophthalic acid linkers in presence or absence of auxiliary ligands, under various hydrothermal conditions; (ii) to apply the synthesized coordination polymers as heterogeneous catalysts for the Knoevenagel condensation reaction of different aldehydes with an active methylene compound. Hence, we now report the synthesis of the novel 5-acetamidoisophthalic acid (H2L) species which is then applied to the preparation, under hydrothermal conditions, of new Zn(II)-coordination polymers. The strategy successfully yielded the polymeric materials [Zn(L)(H2O)2]n (1), [Zn(L)(H2O)2]n·n(formamide) (2), [Zn(L)(H2O)2]n·n(N-methylformamide) (3), [Zn(L)(H2O)(formamide)]n (4) and [Zn3(L)2(4,4’bipyridine)3]n·2n(DMF)·2n(H2O) (5). The obtained frameworks act as heterogeneous catalysts in the Knoevenagel condensation reaction of malononitrile with various aldehydes.

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Experimental The synthetic work was performed in air. All the chemicals were obtained from commercial sources and used as received. The infrared spectra (4000–400 cm-1) were recorded on a Bruker Vertex 70 instrument in KBr pellets; abbreviations: s = strong, m = medium, w = weak, bs = broad and strong, mb = medium and broad. The 1H and 13C NMR spectra were recorded at ambient temperature on a Bruker Avance II + 300 (UltraShieldTMMagnet) spectrometer operating at 300.130 and 75.468 MHz for proton and carbon13, respectively. The chemical shifts are reported in ppm using tetramethylsilane as the internal reference; abbreviations: s = singlet, d = doublet, t = triplet, q = quartet. Carbon, hydrogen and nitrogen elemental analyses were carried out by the Microanalytical Service of the Instituto Superior Técnico. Electrospray mass spectra (ESI-MS) were run with an ion-trap instrument (Varian 500-MS LC Ion Trap Mass Spectrometer) equipped with an electrospray ion source. For electrospray ionization, the drying gas and flow rate were optimized according to the particular sample with 35 p.s.i. nebulizer pressure. Scanning was performed from m/z 100 to 1200 in methanol solution. The compounds were observed in the positive mode (capillary voltage = 80–105 V). Thermal properties were analyzed with a Perkin-Elmer Instrument system (STA6000) at a heating rate of 2˚C min-1 under a dinitrogen atmosphere. Powder Xray diffraction (PXRD) was conducted in a D8 Advance Bruker AXS (Bragg Brentano geometry) theta2theta diffractometer, with copper radiation (Cu Kα, λ = 1.5406 Å) and a secondary monochromator, operated at 40 kV and 40 mA. Flat plate configuration was used and the typical data collection range was between 5˚ and 40˚. Synthesis of 5-acetamidoisophthalic acid (H2L) A 1.81 g (10 mmol) portion of 5-aminoisophtalic acid was dissolved in 20 mL of acetic anhydride and the reaction mixture was refluxed for 4 h at 80 ˚C, after which 20 mL of water were added and the solution further heated until boiling. After cooling, the obtained white solid product of 5-acetamidoisophthalic acid (H2L) was filtered off and washed with water until total removal of acetic acid. Yield: 85% (1.54 g). FT-IR (KBr, cm-1): 3546 (s), 3451(s), 3130(mb), 1723(s), 1673(s), 1616(s), 1583(s), 1459(m), 1407(m), 1372(w), 1338(m), 1308(s), 1255(s), 1208(s), 1130(w), 1109(w), 1031(m), 1001(w), 989(w), 924(s), 806(s), 782(m), 755(s), 676(s), 608(w), 587(w), 542(w), 505(s). 1H-NMR (DMSO-d6): 10.29 (1H, s, -NH), 8.38 (2H, s, Ar-H), 8.13 (1H, s, Ar-H), 2.0 (3H, s, -CH3). 13C-NMR (DMSO-d6): 169.5, 166.9, 140.2, 132.0, 124.8, 123.8, 24.3. MS (ESI): m/z: 246.0 [M+Na]+. Synthesis of 1 A mixture of Zn(NO3)2.6H2O (29 mg, 0.10 mmol) and H2L (22 mg, 0.10 mmol) was dissolved in 1 mL of DMF and methanol (1 : 1). The resulting mixture was sealed in an 8 mL glass vessel and heated at 75˚C for 48 h. Subsequent gradual cooling to room temperature (0.2 ˚C min-1) afforded colorless crystals of 1. Yield: 54% (based on Zn). Anal. Calcd. for C10H11NO7Zn (M = 322.59): C, 37.23; H, 3.44; N, 4.34; Found: C, 36.02; H,3.65; N, 4.02. FT-IR (KBr, cm-1): 3351 (bs), 1680 (s), 1634 (s), 1562 (s), 1420 (m), 1364 (s), 1326 (m), 1282 (m), 1238 (m), 1192 (w), 1103 (m), 1031 (m), 987 (w), 952 (m), 815 (s), 772 (s), 737(m), 538 (w), 460 (m).



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Synthesis of 2 Zn(NO3)2.6H2O (29 mg, 0.10 mmol) and H2L (22 mg, 0.10 mmol) were dissolved in 1 mL of formamide, sealed in a capped glass vessel and heated to 75˚C for 48 h. Subsequent gradual cooling to room temperature (0.2 ˚C min-1) afforded colorless crystals of 2 in ca. 77% yield (based on Zn). Anal. Calcd. for C11H14N2O8Zn (M = 367.63): C, 35.94; H, 3.84; N, 7.62; Found: C, 36.11; H, 4.01; N, 7.45. FT-IR (KBr, cm-1): 3300 (bs), 2882 (w), 1679 (s), 1619 (s), 1580 (m), 1489 (s), 1451 (s), 1424 (m), 1352 (m), 1283 (m), 1221 (m), 1106 (w), 1069 (m), 1045 (m), 915 (w), 814 (s), 777 (s), 730 (m), 642 (m), 620 (m), 531 (m). Synthesis of 3 A mixture of Zn(NO3)2.6H2O (29 mg, 0.10 mmol) and H2L (22 mg, 0.10 mmol) was dissolved in 1 mL Nmethylformamide. The resulting mixture was sealed in an 8 mL glass vessel and heated at 75 ˚C for 48 h. It was subsequently cooled to room temperature (0.2 ˚C min-1), affording plate-like colorless crystals of 3. Yield: 68% (based on Zn). Anal. Calcd. for C12H16N2O8Zn (M = 381.66): C, 37.77; H, 4.23; N, 7.34. Found: C, 37.53; H, 3.92; N, 7.10. FT-IR (KBr, cm-1): 3367 (bs), 2883 (w), 1683 (s), 1613 (s), 1580 (m), 1560 (m), 1490 (s), 1417 (m), 1390 (s), 1347 (s), 1275 (w), 1216 (w), 1100 (w), 1072 (m), 1032 (m), 809 (s), 782 (s), 725 (s), 642 (m), 531 (m). Synthesis of 4 A mixture of Zn(NO3)2.6H2O (58 mg, 0.20 mmol) and H2L (22 mg, 0.10 mmol) was dissolved in 1 mL of formamide. The resulting mixture was sealed in an 8 mL glass vessel and heated at 85 ˚C for 48 h. It was subsequently cooled to room temperature (0.2 ˚C min-1), affording colorless crystals of 4. Yield: 72% (based on Zn). Anal. Calcd. for C11H12N2O7Zn (M = 349.60): C, 37.79; H, 3.46; N, 8.01. Found: C, 36.88; H, 3.01; N, 8.10. FT-IR (KBr, cm-1): 3373 (bs), 3177 (bs), 2916 (w), 1698 (s), 1637 (s), 1562 (s), 1444 (s), 1419 (m), 1384 (s), 1329 (s), 1283 (m), 1257 (w), 1112 (m), 1085 (m), 1034 (m), 935 (m), 799 (s), 782 (s), 725 (s), 634 (m), 542 (m), 474 (m). Synthesis of 5 A mixture of Zn(NO3)2.6H2O (29 mg, 0.10 mmol), H2L (22 mg, 0.10 mmol) and 4,4’-bipyridine (15.6 mg, 0.10 mmol) was dissolved in 1 mL of DMF and methanol (1 : 1). A white precipitate was obtained when 0.5 mL of 28% aqueous ammonia solution was added to this reaction mixture. The precipitate was dissolved upon the addition of additional 0.5 mL of 28% aqueous ammonia solution. Then, the resulting mixture was sealed in an 8 mL glass vessel and heated at 75 ˚C for 48 h. It was subsequently cooled to room temperature (0.2 ˚C min-1), affording plate-like colorless crystals of 5. Yield: 61% (based on Zn). Anal. Calcd. for C58H58N10O18Zn3 (M = 1379.28): C, 50.51; H, 4.24; N, 10.16. Found: C, 49.83; H, 4.92; N, 10.09. FT-IR (KBr, cm-1): 3421 (bs), 2962 (w), 1676 (s), 1612 (s), 1571 (s), 1490 (w), 1417 (m), 1385 (s), 1282 (w), 1249 (w), 1217 (w), 1072 (m), 1047 (w), 809 (s), 783 (s), 717 (w), 642 (m), 532 (w).

Crystal structure determinations X-ray quality single crystals of the compounds were immersed in cryo-oil, mounted in a nylon loop and measured at room temperature (1-5). Intensity data were collected using a Bruker APEX-II PHOTON 100 diffractometer with graphite monochromated Mo-Kα (λ 0.71069) radiation. Data were collected using phi and omega scans of 0.5° per frame and a full sphere of data was obtained. Cell parameters were 4 ACS Paragon Plus Environment

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retrieved using Bruker SMART80-82 software and refined using Bruker SAINT80 on all the observed reflections. Absorption corrections were applied using SADABS.80 Structures were solved by direct methods by using the SHELXS-97 package81 and refined with SHELXL-97.81 Calculations were performed using the WinGX System–Version 1.80.03.82 The hydrogen atoms attached to carbon atoms and to the nitrogen atoms were inserted at geometrically calculated positions and included in the refinement using the riding-model approximation; Uiso(H) were defined as 1.2Ueq of the parent nitrogen atoms or the carbon atoms for phenyl and methylene residues, and 1.5Ueq of the parent carbon atoms for the methyl groups. The hydrogen atoms of coordinated water molecules were located from the final difference Fourier map and the isotropic thermal parameters were set at 1.5 times the average thermal parameters of the belonging oxygen atoms. Least square refinements with anisotropic thermal motion parameters for all the non-hydrogen atoms and isotropic ones for the remaining atoms were employed. Crystallographic data are summarized in Table S1 (Supplementary Information file) and selected bond distances and angles are presented in Table S2. CCDC 1046910-1046914 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Results and Discussion Syntheses and Characterization The hydrothermal reaction, at 75oC, of 5-acetamidoisophthalic acid (H2L) with zinc(II) nitrate hexahydrate (in stoichiometric amounts) in the presence of dimethyl formamide and methanol mixture, formamide or N-methylformamide leads to the formation of [Zn(L)(H2O)2]n (1), [Zn(L)(H2O)2]n·n(formamide) (2) or [Zn(L)(H2O)2]n·n(N-methylformamide) (3), respectively (L = 5acetamidoisophthalate) (Scheme 1). When the hydrothermal reaction of H2L with zinc(II) nitrate hexahydrate is performed in a different molar ratio (1:2) and at a higher temperature (85oC), [Zn(L)(H2O)(formamide)]n (4) is formed (Scheme 2). In the presence of 4,4’-bipyridine and in a mixture of DMF and methanol, [Zn3(L)2(formate)(4,4’-bipyridine)3]n·2n(DMF)·2n(H2O) (5) is obtained (Scheme 2). During the hydrothermal processes, DMF was hydrolyzed to dimethylamine and formic acid, the latter coordinating the metal in the basic form.



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N- Zn( M e NO th 3 ) yl f 2 . 6 or H m 2O am id e

O 2 6H ) 2. OH 3 e O (N , M Zn MF D

Scheme 1 Synthesis and structural representation (repeating units) of 1 – 3. Due to their insolubility in common solvents, the NMR spectra of these frameworks could not be run, but they were characterized by IR spectroscopy, single crystal X-ray diffraction, thermogravimetric and elemental microanalysis.

Scheme 2 Synthesis and structural representation (repeating units) of 4 and 5. In the IR spectra, the characteristic strong bands of coordinated carboxylate groups in 1-5 appear at 1580–1562 cm-1 and 1385-1326 cm-1 for the asymmetric and the symmetric stretching wavenumbers, respectively. The bands in the regions of 1637–1612 cm-1 and 1424–1417 cm-1 are attributed to the C=C


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stretching frequency of the aromatic rings,83,84 and those at 1698-1676 cm-1 to the C=O of the amide group.

Crystal structure analysis Compounds 1-5 crystallize in space groups P21 (1), P21/m (2), P-1 (3 and 4) and C2/c (5). Compound 1 has a left-handed helix, while 2 and 3 have zig-zag type 1D chains; compound 4 features a 1D chain of fused 16- and 8-membered metallacycles and 5 a 2D architecture with 4, 4’-bipyridine as secondary ligand.

Figure 1 A structural unit of the helical type 1D chain of 1 with partial atom labelling scheme. The structural units of 1, 2 and 3 are similar. These polymers are pseudo-polymorphic species owing to the absence (in 1) or presence of solvent molecules (in 2 and 3). The asymmetric units of 1-3 contain one zinc(II) ion, one L2- ligand, two (or only one, in 2) coordinated water molecules and one non-coordinated molecule of formamide (in 2) or N-methylformamide (in 3) (Scheme 1). The asymmetric unit of 4 also contains one L2- ligand and one the Zn2+ ion and, additionally, one coordinated water and one coordinated formamide molecules, while that of 5 encompasses one L2- ligand and one formate moiety, two zinc cations, one and a half bypiridine and a non-coordinated DMF and water molecules.

A

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Figure 2 (A) Left-handed helical structure of 1. (B) Node-and-linker-type descriptions of the 1D coordination framework in 1, the metal nodes being represented in green and the linkers in pink color. The Zn2+ ions assume tetrahedral geometries in 1 and 2 (τ4 = 0.93 and 0.83, respectively),85 trigonal bipyramidal in 3 and 4 (τ5 = 0.54 and 0.62, respectively)86 and octahedral in 5 where Zn1 is in a more distorted environment compared to Zn2, as demonstrated by the corresponding quadratic elongations and angle variances (1.073 and 181.20 º2 against 1.001 and 2.32 º2, in this order).87

A

B

C Figure 3 Crystal structures of 2 (A) and 3 (B) with partial atom labelling schemes. (C) Node-and-linkertype descriptions of the 1D coordination frameworks in 2 and 3, the metal nodes being represented in green and the linkers in pink color.

The coordination types of the carboxylate groups in 1–5 are diverse, assuming the non-bridging monodentate mode in 1 and 2. This type of linkage also occurs in 3 and 4 with, additionally, the chelating bidentate fashion in the former and the bridging bidentate (syn-syn-μ-carboxylate) in the latter. The bridging bidentate chelation mode (syn-syn-type) is the one present in 5. The coplanarity of the carboxylate groups and the phenyl ring in L2‒ are worth to be mentioned being expressed by the CaromaticCaromaticCcarboxylateO (hereafter denoted by CCCO) torsion angles. In 1 this 8 ACS Paragon Plus Environment

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parameter assumes values of 129.2(4)º and 178.7(4)º thus giving rise to ZnOCO torsion angles of 23.3(5)º and 11.0(5)º, in this order, ultimately resulting in the helical nature of this polymer (Fig. 2). The CCCO and the ZnOCO torsion angles in 2 are of 180º and 0.0º, respectively, the reason why this 1D polymer is markedly flat. Polymers 3 – 5 present CCCO and ZnOCO torsion angles that vary in the ranges of 166.9(3) – 179.1(2)º and 3.1(3) – 18.2(6)º, respectively. Concerning the acetamido CaromaticNCC torsion angles, they adopt values higher than 171.7(4)º evidencing the approximate coplanarity (precise, in the case of 2) between this group and the phenyl ring. The Zn-O bond distances are in the usual range of 1.943(2) – 2.630(4) Å, the highest value only found in 5 and resulting from the chelating bidentate coordination mode of formate, but still within the sum of the Van der Waals radii of Zn and O.

A

B Figure 4 Hydrogen bonded networks of 2 (A) and 3 (B) with hydrogen bonding interaction drawn in black dotted lines. N-methylformamide molecules (in B) are represented in spacefill model.



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O O HN

O H2O

O O O NH2 H2O Zn O

O Zn

O

O NH2

NH2

H2O

Zn

NH2

Zn O

O O

O O NH

OH2 O

O

O

O O

A

B

C

D Figure 5 Perspective view with partial atom labelling scheme (A) and schematic representation (B) of a fragment of polymer 4. (C) One dimensional structure of complex 4 running along the crystallographic c axis. (D) Node-and-linker-type descriptions of 4, the metal nodes being represented in green and the linkers in pink color.

In 4 the metal cations are involved in alternating condensed C2O4Zn2 and C10O4Zn2 metallacycles that construct a 1D polymeric assembly (Figure 5C). In 5, the μ3-L2‒ and the μ-formate ligands bridge the Zn atoms and form ~Zn1-(μ-COO)L-(μ-O)formate-Zn2(μ-COO)L-(μ-O)formate-Zn1~ trinuclear cores which are further inter-connected giving 16-membered C10O4Zn2 rings, thus generating wave like chain motifs parallel to the ac plane (Figure 6B). These are further extended into a 2D metal-organic layer by means of the µ-4,4’-bypiridine. The packing view of 5 is characterized by channels along the crystallographic a axis (Figure 6B) with approximate dimension of 11.39 x 7.59 Å2, which are occupied by DMF and water molecules.


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The minimum Zn···Zn separation in these polymers vary in the order 3.6878(6) (4) < 3.7905(5) (5) < 4.3935(5) (2) < 4.5627(5) (3) < 6.3870(7) (1) Å.

A

B

C Figure 6 (A) Trinuclear secondary building block unit of the coordination polymer 5 with partial atom labelling scheme. (B) 2D packing diagram of 5 (water molecules are represented as spacefill model). (C) Node-and-linker-type descriptions of 5, the metal nodes being represented in green, the linkers in pink, 4,4’-bipyridine in red and formate anion in blue color.

Non-covalent interactions (O–H···O, N–H···O and C–H···O. Table S2, supplementary information) are found in 1 – 5, stabilizing the structures and extending them to 3D. The 1D chains of 1 are interconnected via intricate water···carboxylate contacts; the 1D chains of 2 are involved in a 2D network (in the ac plane) by means of NH···O (acetamido···formamide, formamide··· acetamido and formamide···carboxylate) interactions, ultimately expanding to the third dimension by means of 11 ACS Paragon Plus Environment


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water···carboxylate contacts; the 1D chains of 3 are also tangled in OH···O (water···methylformamide, water···carboxylate and water···acetamido) and in NH···O (acetamido···methylformamide and methylformamide···carboxylate) contacts. The non-covalent interactions in 4 (Table S2) are also of the expected types of OH···O (water···acetamido and water···carboxylate) and N-H···O (formamide···carboxylate formamide···acetamido and acetamido···formamide). The metal-organic network of 5 is reinforced via intermolecular H-bonds (Table S2) involving the crystallization water molecules which simultaneously act as donors (to carboxylate groups) and as acceptors (from the acetamido groups). Structures 1 – 3 have similar types of topologies88,89 and can be represented as 2-connected uninodal nets (Figure 2B and 3C) with topological type 2C1, 4 features a 3-connected uninodal net with a topological type SP1-periodic net (Figure 5D) and 5 reveals a 2,2,2,3,5,6-connected hexanodal net.

Catalytic activity of the Zn coordination polymers in the Knoevenagel condensation The Knoevenagel condensation (Scheme 3) of aldehydes with active methylene compounds is one of the most useful C–C bond forming reactions having wide applications for the synthesis of fine chemicals.63-76 This condensation is generally catalyzed by bases or Lewis acids and widely studied in homogeneous systems.90,91 Only a few studies have employed heterogeneous catalysts including metal organic frameworks.92-96 Recently, Kitagawa et al.73 and Zhou et al.76 reported that amide functionalized MOFs are quite effective for Knoevenagel condensation. Besides that, some Zn-MOFs can also catalyze this condensation in a heterogeneous way.35,64 Thus, the development of suitable MOFs or coordination polymers based catalysts for this reaction is a topic of great interest.97,98 It is known that base and acid sites can catalyze this reaction and that bifunctional catalysts (e.g., amide functionalized MOFs) with adequate acid–base pairs are more active than either purely acid or base catalysts.99-101 Recently, a few amide functionalized MOFs and Zn-MOFs were separately reported to act as catalysts for the Knoevenagel condensation. Thus, our idea was to produce zinc coordination polymers from an amide functionalized ligand and test their catalytic behaviour on this type of reaction. Our newly synthesized zinc(II) coordination polymers have both a Lewis acid (Zn2+) and a basic centre (amide group), and thus they possibly can act as bifunctional catalysts for this condensation. Moreover, our zinc polymers (1-5) have different structural dimensionalities (1D and 2D) and orientations. For example, compounds 1-3 are pseudo-polymorphic supramolecular isomers, having similar 1D type structures but with different structural arrangements and packing. Hence, it would be interesting to see how the different structural features affect the catalytic behaviour. To our knowledge only a few examples of supramolecular isomers are known which act as catalysts for the Knoevenagel condensation reactions.75 Thus, we have tested the catalytic activity of the coordination polymers 1-5 as solid heterogeneous catalysts in the Knoevenagel condensation of malononitrile with various aldehydes. In a typical reaction, a mixture of benzaldehyde (51 μL, 0.50 mmol), malononitrile (66 mg, 1.0 mmol) and Zn-catalyst (5 mg of 1, 5.5 mg of 2, 6 mg of 3, 5 mg of 4 or 21 mg of 5, 3 mol%) was placed in a capped glass vessel, and then 1 mL THF was added into it. The mixture was heated at 40 ˚C for 1.5 h, and subsequently quenched by centrifugation and filtration at room temperature. The filtrate was evaporated in vacuum to give the crude product. The residue was dissolved in CDCl3 and analyzed by 1H NMR. The 1H-NMR spectra and the 12 ACS Paragon Plus Environment

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calculation of the yield for compound 3 in the Knoevenagel reaction is presented in the supporting information (Figure S3). By using benzaldehyde as a test compound, we found that 3 led to the highest product yield, as compared to the other polymers after the same reaction time and at the same temperature. Consequently, the optimization of the reaction conditions (temperature, reaction time, amount of catalyst, solvent) was carried out in a model malononitrile–benzaldehyde system with 3 as the catalyst (Scheme 2 with typical reaction conditions; Table 1).

CN CHO

NC + NC

CN

Catalyst THF, 40oC, 90 min

Scheme 3 Knoevenagel condensation reaction of benzaldehyde with malononitrile Under the above typical conditions (3 mol% of solid 3 at 40˚C with THF as the best solvent, as indicated below), a conversion of 98% of benzaldehyde into 2-benzylidenemalononitrile is reached (entry 5, Table 1) after 1.5 h. With compounds 1, 2, 4 and 5, yields of 91%, 89%, 78% and 61% were obtained, respectively. Hence, our five coordination polymers activities follow the order of 3 > 1 ≈ 2 > 4 > 5. The relationship between structure and catalytic activity in the present study is not clearly understood. However, the highest conversation shown by the compound 3 may be due its packing arrangement where the 1D chains are hydrogen bonded with one another and form a 2D sheet. The N-methyl formamide molecules trapped inside may eventually assist the substrate molecules (benzaldehyde and malononitrile) to form acid base pairs with metal centres. In the other cases (for 1, 2, 4 and 5) the tight packing doesn’t support the above hypothesis. The plot of yield versus time for the Knoevenagel condensation reaction of benzaldehyde and malononitrile with compound 3 is presented in Figure 8A.

A

B 13 ACS Paragon Plus Environment


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Figure 8 Plots of yield vs. time for the Knoevenagel condensation reaction of benzaldehyde and malononitrile in the presence of catalyst 3. (A) In THF (black curve) and upon separation of catalyst 3 (red curve). (B) In the presence of several solvents.

Blank reactions were tested with benzaldehyde in the absence of catalyst, at 40˚C in THF, and no conversion of aldehyde into 2-benzylidenemalononitrile was detected, after 1.5 h (entry 19, Table 1). The reaction also did not take place by using the free ligand H2L instead of catalyst 3 (entry 20, Table 1). We have also checked the reactivity of different zinc(II) salts and ZnO in THF and the obtained reaction yields are much lower, in the range of 22-43% (entries 21-25, Table 1). Moreover, we have performed this condensation reaction by using 1:1 mixtures of ligand H2L and Zn(II) salts and the obtained yields lie between 21 and 30% (entries 26-28, Table 1), not supporting a catalytic cooperative role. In addition, we have also tested the activity of various metal oxides (different particle size) towards the same reaction under similar conditions and the observed yields are in the 9-53% range (entries 29-34, Table 1). The obtained results indicate that our catalyst 3 is much better than the tested metal oxides under equivalent reaction conditions. Table 1 Optimization of the parameters of the Knoevenagel condensation reaction between benzaldehyde and malononitrile with 3 a as the catalyst b c Entry Catalyst Time (h) Amount of T (˚C) Solvent Yield (%) TON Catalyst (mol%)

1 2 3 4 5

3 3 3 3 3

0.25 0.5 0.75 1 1.5

3.0 3.0 3.0 3.0 3.0

40 40 40 40 40

THF THF THF THF THF

33 53 63 76 98

11 18 21 25 33

6 7 8

3 3 3

1.5 1.5 1.5

1.0 5.0 7.0

40 40 40

THF THF THF

77 97 98

77 18 14

9 10 11

3 3 3

1.5 1.5 1.5

3.0 3.0 3.0

40 40 40

CH3CN CH2Cl2 MeOH

86 70 75

29 23 25

12 13 14

3 3 3

1.5 6.0 1.5

3.0 3.0 3.0

RT (15 C)

25

THF THF THF

40 97 88

13 32 29

15 16 17 18

1 2 4 5

1.5 1.5 1.5 1.5

3.0 3.0 3.0 3.0

40 40 40 40

THF THF THF THF

91 89 78 61

30 29 26 20

19

Blank

1.5

-

40

THF

No Reaction

-

o o

RT (15 C)


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20 21 22 23 24 25 26


H2L Zn(NO3)2.6H2O Zn(OAc)2.2H2O ZnSO4.7H2O ZnCl2 ZnO

H2L +

1.5 1.5 1.5 1.5 1.5 1.5 1.5

3.0 3.0 3.0 3.0 3.0 3.0 3.0

40 40 40 40 40 40 40

THF THF THF THF THF THF THF

No Reaction 41 39 22 29 43 24

14 13 7 10 14 8

1.5

3.0

40

THF

21

7

Zn(NO3)2.6H2O

27

H2L + ZnSO4.7H2O H2L + ZnCl2

28 1.5 3.0 40 THF 30 10 29 Al2O3 1.5 3.0 40 THF 21 7 30 V2O5 1.5 3.0 40 THF 25 8 31 CdO 1.5 3.0 40 THF 53 17 32 MoO2 1.5 3.0 40 THF 13 4 33 PbO2 1.5 3.0 40 THF 9 3 34 HgO 1.5 3.0 40 THF 26 8 a Reaction conditions: 3.0 mol% of catalyst 3, solvent (THF) 1 mL, malononitrile (66 mg, 1.0 mmol) and benzaldehyde (0.50 mmol). bCalculated by 1H NMR. c Number of moles of product per mole of catalyst.

The effects of temperature, catalyst amount and solvents were also tested. The increase of the amount of catalyst 3 from 1.0 to 3.0 mol% enhances the product yield from 77 to 98%, but further increase amount of catalyst leads to no significant increase in catalytic activity (entries 6–8, Table 1). To select the most suitable solvent, experiments with various solvents (CH3CN, CH2Cl2, THF and MeOH) have been carried out with compound 3, and the corresponding plots of yield vs. time are presented in Figure 8B. The results indicate that THF (yield of 98%) is the best solvent, whereas the worst one is CH2Cl2 (70% yield) for the same reaction time (1.5 h) (Table 1, entries 5 and 10). In methanol or acetonitrile, the yield of 86% or 75%, respectively, was obtained (Table 1, entries 9 and 11) after 1.5 h. Increasing the temperature from 15 to 40˚C improved the 2-benzylidenemalononitrile yield from 40 to 98% (entries 12 and 5, Table 1). However, we have obtained 97% of yield in 6 h at 15˚C (room temperature) (entry 13, Table 1). Table 2 Knoevenagel condensation reaction of various aldehydes with malononitrile with catalyst 3a Entry compound Yieldb(%) TONc 1

100

33

2

26

8

3

94

31

4

70

23

5

86

28



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6

69

Page 16 of 24

23

a

Reaction conditions: 3.0 mol% of catalyst 3, solvent (THF) 1 mL, malononitrile (66 mg, 1.0 mmol) and aldehyde (0.50 mmol); reaction time of 1.5 h. bCalculated by 1H NMR. c Number of moles of product per mole of catalyst. We also investigated the catalytic activity of 3 with different types of substituted aromatic aldehydes in the reaction with malononitrile. The results are summarized in Table 2. p-Nitrobenzaldehyde produced the maximum yield (100%), while the lowest one (26%) was obtained for p-methoxybenzaldehyde, suggesting that an electron-withdrawing substituent promotes the reactivity, in contrast to an electrondonating moiety, what may be related with an increase of the electrophilicity of the substrate in the former case. In order to perform the catalyst recycling experiments, the used catalyst 3 (separated by centrifugation of the supernatant solution) was washed with THF and dried in air. It was then reused for the Knoevenagel condensation reaction as described above. The catalyst 3 was recycled in three consecutive experiments, and its activity remained essentially the same (Figure S1). FT-IR spectra of catalyst 3 taken before and after the reaction suggest that the structure of the solid was retained (Figure S2A). We have also performed powder X-ray diffraction with internal standard (NaCl) of catalyst 3 before and after the condensation reaction and observed only a slight difference (2%) in peak intensity after the reaction (Figure S2B). These experiments are indicative that the material essentially remains intact during the catalytic process. To further verify the heterogeneity of the system, a procedure similar to that of Sheldon et al. was followed.102 We performed a controlled experiment, removed the catalyst by centrifugation and the catalyst free reaction solution was kept under the same conditions and monitored by NMR along the time to determine whether the metal component dissolved appreciably from the solid structure of our catalyst. In our experiment, the catalyst 3 was removed by filtration from the mixture once the conversion was up to ca. 53% (after 30 min reaction time), whereupon the supernatant fluid was stirred for an additional 1 h under the same reaction conditions. As shown in Fig. 8A, after separation of the solid catalyst, no conversion of benzaldehyde is observed. These results demonstrate that the catalysis is heterogeneous in nature. Additionally, the filtrate solution, after the separation of the catalyst, was evaporated to dryness and the amount of zinc determined, being only 0.02% of the amount used in the reaction, thus ruling out any significant leaching of the catalyst. There are some reports on coordination polymers which are catalysts for this kind of reaction,63-76 and a comparison with our catalyst 3 is shown in Table 3. The obtained yields in our system are usually higher than for the other reported zinc(II) frameworks in which the reaction of benzaldehyde and malononitrile, after a longer time and at a higher temperature, leads to an overall yield that is lower or identical to ours (Table 3, entries 2,3).69,70 Similarly, a 3D metal–organic framework built from nickel(II) ions as connectors and methanetetrabenzoate ligands produced 78% yield after 6 h at 130oC (Table 2, entry 4).71 Moreover, when we performed the reaction at room temperature with our catalyst 3, a 97% yield was reached only in 6 h (Table 3, entry 6), which is a much lower time than those for other 16 ACS Paragon Plus Environment

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reported Cd(II) and Pb(II)-MOFs (Table 3, entries 7 and 8).73,74 In comparison with other catalysts, ours have the advantages of being rather cheap, easy-to-prepare and highly active at low temperature. Moreover, they are recyclable without appreciable loss of activity. Table 3: A comparison of catalytic activity of various coordination polymers in the Knoevenagel condensation reaction of benzaldehydes and malononitrile Solvent/Temp/Time

Aldehydes

3

THF/ 40°C/ 1.5 h

2

Zn2dobdc

3 4

Entry

Catalyst

1

5 6 7 8 9 10

Yield (%)

Reference

Benzaldehyde

98

This work

Toluene/70°C/ 24 h

Benzaldehyde

77

69

Zn2(tpt)2(2-atp)I2

Ethanol/60°C/ 2 h

Benzaldehyde

99

70

{[Ni4(μ6-MTB)2(μ2H2O)4(H2O)4]·10DMF·11H2O}n [Tb(BTATB)(DMF). 2(H2O)]·DMF·2H2O 3 [Cd(4-btapa)2(NO3)2]. 6H2O. 2DMF Pb(cpna)2.2DMF.6H2O 3 {[Cd(NH2_bdc)(bphz)0.5]·DMF·H2O}n

p-xylene/ 130°C/ 6 h

Benzaldehyde

78

71

CH3CN/ 60°C/ 24 h

Benzaldehyde

99

72

THF/ RT/ 6 h C6H6/RT /12 h CH3CN/ RT/ 24 h THF/ 40°C/ 1.5 h CH3CN/ 60 °C/ 6 h

Benzaldehyde Benzaldehyde Benzaldehyde 4-nitrobenzaldehyde 4-nitrobenzaldehyde

97 98 100 100 99

This work 73 74 This work 75

dobdc = 2,5-dioxidoterephthalate; MTB = methanetetrabenzoate; DMF = N,N’-dimethylformamide; BTATB = 4,4′,4″-(benzene-1,3,5triyltris(azanediyl))tribenzoate; 4-btapa = 1,3,5-benzene tricarboxylic acid tris[N-(4-pyridyl)amide]; cpna = N-(4-carboxyphenyl)isonicotinamide 1-oxide; NH2_bdc = 2-aminobenzenedicarboxylic acid; bphz=1,2-bis(4-pyridylmethylene) hydrazine; tpt = tris (4-pyridyl) triazine; 2-atp = 2aminoterephthalate.

CN NC

CHO

H + H2O

H C

[Zn

C

CN CN

H

[Zn

O]

CN

CN H

H

O

+ NC

O]

C

O

H

C

C N

[Zn

H O]

Scheme 4: Proposed catalytic cycle for the Knoevenagel condensation reaction catalyzed by 1-5. A possible mechanism for the Knoevenagel condensation catalyzed by 1-5 is presented in Scheme 4, on the basis of reported proposals.103 In our coordination polymers, the zinc centre (Lewis acid site) interacts with the carbonyl group of the benzaldehyde, producing a polarization of this group and thus increasing the electrophilic character of the carbonylic carbon atom. This polarization favours the attack 17 ACS Paragon Plus Environment


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from the malononitrile. The interaction of a cyano group of malononitrile with the Lewis acid (zinc site) increases the acidity of the methylene group which enhances the deprotonation of malononitrile. The basic sites (carboxylate-O or amide-O) can abstract the proton from the methylenic group to generate the corresponding nucleophilic species which attacks the carbonyl group of benzaldehyde with C-C bond formation (aldol condensation) and dehydration. If the acid and basic sites of the bifunctional catalysts are well balanced, a concerted and synergic action can take place.

Conclusion Five zinc(II) coordination polymers constructed with 5-acetamidoisophthalic acid (H2L) were synthesized. They exhibit 1D (1, 2, 3 and 4) and 2D (5) structures, on account of the various coordination modes of the ligand. Single-crystal X-ray diffraction analysis revealed that 1, 2 and 3 are pseudo-polymorphic supramolecular isomers, with 1 having an helical assembly, but 2 and 3 exhibiting similar zig-zag type structures containing different guest molecules (formamide in 2 and methylformamide in 3). Compound 4 is a 1D coordination polymer with fused 8-membered and 16-membered dimetallic rings, whereas compound 5 shows a two dimensional layered structure with trimetallic cores. Zinc(II) coordination numbers and geometries vary from tetrahedral (in 1 and 2) to trigonal bipyramidal (in 3 and 4) and to octahedral (in 5). These coordination polymers act as heterogeneous catalysts for the Knoevenagel condensation of aldehydes with malononitrile, at low temperature. Among the five coordination polymers, 3 is the most effective catalyst for this condensation reaction. It effectively catalyzes the reaction of various aldehydes and malononitrile producing the corresponding benzylidenemalononitriles in high yields, which depend on the electrophilicity of the substrate. Moreover, the catalyst is stable and recyclable. Moreover, the present study displays an example of pseudo-polymorphic supramolecular isomers acting as heterogeneous catalysts for the Knoevenagel condensation. It also presents the dependence of the catalytic properties on the supramolecular structural arrangements. This study also provides further evidence that zinc(II) coordination polymers can be utilized as effective heterogeneous catalysts, under mild conditions, in this important organic reaction. Further explorations into the uses of this catalyst family in other organic transformations, as well as mechanistic investigations, are ongoing.

Supporting Information CIF files and crystallographic data, bond distances, angles and hydrogen bonding tables for all compounds (1-5), thermogravimetric analysis, FT-IR and PXRD before and after catalysis, 1H-NMR spectra after catalysis, curve for recycling experiment. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements This work has been supported by the Foundation for Science and Technology (FCT), Portugal (project UID/QUI/00100/2013). Authors AK and SH express their gratitude to the FCT for post-doctoral fellowships (Ref. No. SFRH/BPD/76192/2011 and SFRH/BPD/78264/2011). Authors MFCGS and AJLP also acknowledge the Russian Science Foundation for support to the synthesis with malononitrile and for the 18 ACS Paragon Plus Environment

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help in organizing the International Laboratory (grant 14-43-00017). Thanks are also due to the Portuguese NMR Network (IST-UL Centre) for access to the NMR facility, and the IST Node of the Portuguese Network of mass-spectrometry (Dr. Conceição Oliveira) for the ESI-MS measurements.

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For Table of Contents Use Only Solvent dependent structural variation of zinc(II) coordination polymers and their catalytic activity in the Knoevenagel condensation reaction Anirban Karmakar,* Guilherme M. D. M. Rúbio, M. Fátima C. Guedes da Silva,* Susanta Hazra, and Armando J. L. Pombeiro* Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049–001, Lisbon, Portugal. E-mail: [email protected]; [email protected]; [email protected]

Synopsis 5-acetamidoisophthalic acid is used to synthesize five new zinc(II) complexes which act as heterogeneous catalysts for the Knoevenagel condensation reaction of aldehydes with an active methylene compound.


Solvent-Dependent Structural Variation of Zinc(II) Coordination

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